Packet-forwarding policies are in place to administer, manage and control access to network resources. Policy-based management employs a policy server to manage the network as a whole. The policy server translates business goals or policies into configurations of network resources and automates the configurations across multiple different network elements and different technologies (e.g. MPLS and Diffserv). The centralized approach ensures policy consistency across multiple network elements.
Multi-protocol label switching (MPLS) is a packet-forwarding technology that gives internet protocol (IP) operators a high degree of control over the paths taken by packets on their networks. MPLS may be used for traffic engineering purposes. Interior gateway protocols (IGP), such as open shortest path first (OSPF) and intra-domain intermediate system to intermediate system routing protocol (IS-IS), route IP packets based only on the destination address and the shortest path to reach the destination. In contrast, MPLS traffic engineering allows administrators to establish routes for certain customers based on information other than the shortest path, such as delay and bandwidth available along the path. Therefore, MPLS can relieve congestion and maximize bandwidth utilization by allowing multiple paths between source and destination.
Constraint based routing (CBR) is an example of MPLS traffic engineering. The operator does not specify the path explicitly but depends on a CBR mechanism that has been implemented in the network to determine the path. With CBR, every router advertises traffic-engineering attributes (e.g., maximum bandwidth, unreserved bandwidth, etc.) for its interfaces to all other routers. OSPF and IS-IS protocols have been extended for that purpose. As a result of the advertisement, every router obtains a traffic-engineering database in addition to the regular routing database. When a label switch path (LSP) with QoS requirements needs to be established, the ingress router computes the optimal path based on the traffic-engineering database and a path selection algorithm. The router then signals the path establishment with a label distribution protocol. The explicit path selected is conveyed through the signaling protocol.